1. Field of the Invention
This invention relates to a three-axis superconducting gravity gradiometer and more particularly to improvements in the gradiometer described in the inventor's prior publications in Proc. 17th Int. Conf. Low Temp. Phys., Kahlsruhe, W. Germany (July, 1984), in Proc. 10th Int. Cryogenic Eng. Conf., Helsinki, Finland (August, 1984) and in IEEE Trans. Magnetics, MAG-21, 411 (March, 1985), the descriptions of which are incorporated by reference into the present disclosure, as are the descriptions of all other prior publications noted hereinafter.
2. Discussion of Background
A three-axis in-line gravity gradiometer measures the three diagonal components of the gravity gradient tensor at the same point in space-time. It can be formed from three orthogonal in-line gravity gradiometers [see Paik, J. Astronaut, Sci. 29, pp. 1-17 (1981)]. A "current-differencing" mode is applied. The following is a brief review of an in-line component gradiometer (i.e., a gradiometer which is sensitive to the diagonal components of the gravity gradient tensor, r.sub.ii), which can be extended to a cross-component gradiometer (i.e., one which is sensitive to an off-diagonal component of the gravity gradient tensor, r.sub.ji,j.GAMMA.i).
An in-line component superconducting gravity gradiometer consists of a pair of spring-mass accelerometers coupled together by a superconducting circuit to measure differential acceleration. As shown schematically in FIG. 1, each accelerometer consists of a superconducting proof mass 10 confined to move along a single axis and a spiral superconducting sensing coil 12 located near the surface of the proof mass 10. An acceleration will cause a displacement of the proof mass 10 which, because of the Meissner effect, will modulate the inductance of the coil 12 at frequencies down to dc. The sensing coil is connected to the input coil 14 of a superconducting quantum interference device (SQUID) amplifier 16 forming a closed superconducting loop. This loop contains a persistent current which couples the mechanical and electrical systems. Since the flux in this loop must remain constant, the change in the inductance of the sensing coil results in a current change through the SQUID input coil 14. In this manner, very small accelerations can be detected.
The following considerations are important for each pair of coupled acceleration transducers:
(1) In order to minimize the contamination of the signal by the SQUID amplifier noise, a very low proof mass resonance frequency in the differential mode is desirable in order to produce, for a given acceleration amplitude, a larger proof mass displacement before it is detected by the superconducting circuit.
(2) The spring used in the suspension should have low loss in order to have lower thermal (Nyquist) noise from the spring.
(3) A precise alignment of the sensitive axes and a high degree of common mode rejection are needed in order to reject the relatively large common accelerations of the gradiometer platform.
The gradiometer discussed in the Proc. 17th Int. Conf. Low Temp. Phys, supra, consists of three pairs of coupled spring-mass type acceleration transducers mounted on six faces of a precision cube. Each pair of acceleration transducers on opposite faces of the cube are coupled passively through a superconducting circuit to measure common and differential accelerations. A gravity gradient signal is measured as the differential acceleration over the baseline between the pair of transducers. A schematic for one transducer is shown in FIG. 2. The center Niobium (Nb) proof mass 10, which is confined to move along a collinear axis by a pair of low-loss cantilever springs 18, displaces in response to an acceleration. Such a displacement modulates the inductance of Nb pancake coils 12 which have stored magnetic flux. Coupling a pair of these transducers in a superconducting circuit and adjusting the stored flux in each loop of this circuit enable an exact, passive and hence noiseless differencing of the accelerations in the form of a supercurrent signal which is measured with a SQUID amplifier.
The superconducting circuit for each single-axis component gravity gradiometer of the inventor's earlier gradiometer is shown in a simplified form in FIG. 3. The coupled motions of the proof masses m.sub.1 and m.sub.2 can be decomposed into a common acceleration mode and a differential acceleration mode with respectively large and small electromechanical spring constants. These spring constants are due to the soft mechanical springs and the coupled circuit of sensing coils (solid line) and "push-pull levitation" coils (dotted line). The main symmetry breaking element in the spring constants of the two acceleration modes are the push-pull levitation coils 22 which lift m.sub.1 and m.sub.2 against Earth's gravity and give a strong spring component to the common mode. In the differential mode, however, the inductances of the levitation coils for the two proof masses change in a complementary manner resulting in no change in the total inductance of these coils, which are connected in series as shown. Therefore, these coils form a zero-frequency spring. The terrestrial environment has vibration noise that is several orders of magnitude larger than the gravity gradient signals of interest. A high resonance frequency is desirable for the common mode in order that the gradiometer is less susceptible to disturbances from terrestrial vibrations. The passive common-mode resonance frequency is over 50 Hz.
In order to compromise between alignment precision and sensitivity, a mechanical cantilever-spring suspension was used, to confine the motion of the proof mass along a straight line. The mechanical suspension provides the convenience of employing mechanical precision to align the sensitive axes of a pair of in-line acceleration transducers along a common collinear direction and to align this common axis along a reference axis of the precision cube. The cantilever springs, which are relatively soft in the bending mode but stiff against stretching, provide the confinement for the motion of the proof mass to a one-dimensional motion. However, the mechanical suspension also raises the resonance frequency of the proof mass and hence sets an unnecessary limit on the sensitivity of the gradiometer. A passive superconducting negative spring, which lowers the resonance frequency without adding amplifier noise, can be used, as discussed hereinafter, to extend the intrinsic sensitivity of the gradiometer. Basic design considerations of a passive superconducting negative spring are described by Parke et al, in the above-noted reference, Proc. 10th Int. Cryogenic Eng. Conf., Helsinki, Finland (1984).
Other prior publications of interest are Paik, "Superconducting Tensor Gravity Gradiometer For Satellite Geodesy and Inertial Navigation", Journal of the Astronautical Sciences, Vol. XXIX, No. 1, pp 1-18, January-March, 1981; Moody et al, "Preliminary Tests of A Newly Developed Superconducting Gravity Gradiometer", Proceedings of 1982 Applied Superconductivity Conference, Knoxville, Tenn. (Nov. 1982); and Paik, "Geodesy and Gravity Experiment in Earth Orbit Using a Superconducting Gravity Gradiometer", IEEE Trans. On Geoscience and Remote Sensing, Vol. GE-23, No. 4, July 1985.